The electrolytic reduction of metals (electrorefining or electrowinning) is carried out in several electrolytic cells, in which electrodes (anodes and cathodes) are loaded in an alternating order. Individual cells are arranged in cell groups by coupling the cells electrically in series by means of a separate contact system. This kind of contact system includes a busbar (so-called partition wall busbar), the task of which is to distribute the electric current evenly from the cathodes of the preceding cell to the anodes of the next adjacent cell.
From the field of electrolytic reduction of metals (electrorefining and electrowinning), there are known busbar systems representing two principal types.
The busbar system of the first main type is characterized by a uniform partition wall busbar. This kind of systems are widely used on the industrial scale in electrolytic plants. One application is known from a so-called Walker busbar system that is presented in the publication U.S. Pat. No. 687,800. There a number of electrolytic cells is arranged to form a cell group, where the cells are separated by a number of partition walls. In each cell, there are arranged in an alternating order a number of anodes and cathodes, so that in each cell, there is a cathode next to each anode. In addition, each individual anode in each cell is positioned in the same line—which in this specification is called the anode line—with the anode of the adjacent cell, and each individual cathode in each cell is positioned in the same line—which in this specification is called the cathode line—with the cathodes of the adjacent cell. A uniform busbar extending along the whole length of the cell is arranged on top of the partition wall between each of two adjacent cells in order to galvanically connect all of the anodes of the cell with all of the cathodes of the adjacent cell. In the publication EP 1095175 B1, the Walker system is developed further by adding equipotential bonding rails for the electrodes. The system is also known by the name “Outotec Double Contact Bus Bar System”. It can be used for alleviating the effect of contact errors between the busbar and the electrodes.
The busbar system representing the other main type is characterized by a so-called segmented partition wall busbar, i.e. there the busbar is not uniform. This kind of segmented intercell busbar system (Optibar) is described in the following scientific articles:                1. /G. A. Vidal, E. P. Wiechmann and A. J. Pagliero, “Technological Improvements in Copper Electrometallurgy: Optibar Segmented Intercell Bars (Patent Pending)”. Canadian Metallurgical Quarterly, Vol. 44, No 2. 2005, 147-154/,        2. /G. A. Vidal, E. P. Wiechmann and A. J. Pagliero, “Performance of Intercell Bars for Electrolytic applications: A Critical Evaluation”. Hydrometallurgy 2003—Fifth International Conference in Honor of Professor Ian Ritchie—Volume 2: Electrometallurgy and Environmental Hydrometallurgy, 2003, 1381-1393./ and        3. /E. P. Wiechmann, G. A. Vidal and A. J. Pagliero, “Current-Source Connection of Electrolytic Cell Electrodes: An Improvement for Electrowinning and Electrorefinery”, IEEE transactions is industry applications, vol. 42, no. 3, May/June 2006, 851-855/.        
The present invention relates to segmented partition wall busbar systems according to the second main type mentioned above, and the Optibar system can be considered as representative of the nearest prior art with respect to the invention at hand.
FIG. 1 illustrates a known Optibar system. A number of electrolytic cells 1 is arranged to form a cell group, where the cells are mutually separated by a number of partition walls 3. In each cell, there are arranged, in an alternating order, a number of anodes A and cathodes C, so that in each cell, next to each anode A there is placed a cathode C, and further so that in each cell, each individual anode A is in the same anode line LA with the anode of the adjacent cell, and that in each cell, each individual cathode C is in the same cathode line LC with the cathode of the adjacent cell. The busbar 4 is arranged on top of the partition wall 3 arranged in between each adjacent cell. The busbar is formed of a row of conductor segments 5 that are separated galvanically from each other. Each conductor segment 5 is arranged to galvanically connect each anode A always with one cathode C of the adjacent cell. In each cell, the anode located in the anode line is galvanically connected by the conductor segment 5 in pairs (as can be seen in FIG. 1) to the cathode in the adjacent cell, located in the adjacent cathode line placed on the same side of the anode line. Thus the electric current proceeds directly from the cathode of the preceding cell to the anode of the following cell. Because each conductor segment in between different cells always proceeds in the same direction, inside the cell group electric current flows in parallel with the imaginary cell diameter (which is in the drawing illustrated by arrows drawn in between the anodes and cathodes; said arrows schematically illustrate the proceeding of current in the electrolyte in between the anode-cathode pairs).
Disturbances that are generally and typically detected in electrolysis are:                contact error between electrode and busbar        irregular electrode intervals (differences in distances between electrodes)        short circuit between anode and cathode        disturbances caused by the electrolyte (for example additive treatment of copper electrolysis).        
The basis for a well functioning electrolysis is that current distribution for individual electrodes in the electrolytic cell is as even as possible, from the beginning of the electrolytic cycle to the end. Now, particularly in the beginning of the electrolytic cycle, the effect of contact errors between the electrodes and the busbar must be minimized. As a consequence of contact errors, for instance the specific energy consumption in the electrolysis and the probability of short circuits is increased. The created short circuits in turn result in a decrease of current efficiency. Also the irregularity in the mass distribution of the cell cathodes is likewise increased. Irregular electrode intervals (distance differences) are mainly due to electrode rifling errors, deviations in electrode thicknesses, bending of electrodes and wrong position in suspension. As a consequence of an irregular electrode interval, the distribution of electrolyte resistance in the cell group is not even. Further, as a consequence of an irregular electrode interval, the probability of short circuits is increased, and the current efficiency is decreased. In case of a short circuit, current proceeds through the short circuit directly from the anode to the cathode. Naturally this results in that the current efficiency is decreased, and the quality of the metal precipitated on the surface of a short circuited cathode is weakened.
A wrong composition of the electrolyte can mean that both the chemical and physical qualities of the metal precipitated on the cathode surface are weakened. The weakening of the physical quality results in an increase of the number of short circuits, and in a decrease of the current efficiency. By means of the structure of the partition wall busbar, it is possible to restrict the effects of the drawbacks caused by the three first types of disturbances.
The advantage of the segmentation of the partition wall busbar in the Optibar style is that it cuts down the short circuit current. Owing to the use of a segmented busbar, the current efficiency in the cell group is good also in case of a short circuit. A good current efficiency is achieved because the segmentation of the busbar restricts the quantity of the electric current that is transferred to the short circuited electrodes.
However, a drawback of the Optibar system is that it causes a remarkable distortion in the distribution of the effective current in the cell group, wherefore the Optibar system is problematic in use. This remarkable phenomenon has not been identified in the above mentioned articles /1/-/3/on the Optibar system, because there the cathode streams are observed by a coarse resistor network analysis. Instead, the articles emphasize the evenness of the current distribution.
The distortion of the effective currents that takes place in the Optibar system is illustrated by FIG. 2 obtained from the FEM simulation model. FIG. 2 illustrates an electrolytic system that is meant for copper electrorefining. FIG. 2 is a schematical illustration of a cell group with 7 cells, where each cell includes 60 electrode intervals, i.e. 31 anodes and 30 cathodes. By FEM model simulation, there is obtained an effective current distribution in a cell group in a so-called ideal i.e. undisturbed situation according to the drawing, without short circuits etc. Here the term ‘effective current’ refers to the current passing through the electrolyte and participating in the metal precipitation process. As was mentioned above, it would be advantageous that the effective current distribution were as even as possible, so that the obtained layer of metal precipitated on the cathodes is evenly thick, i.e. the mass distribution of the cathodes is as even as possible. In the example, the optimal effective current in all electrode intervals of the whole cell group would be for instance 325 A. However, in the Optibar system of FIG. 2, the obtained current distribution range is large, extending from the value 0 A to the value of roughly 700 A, as can be seen from the vertical column on the right-hand side of the Figure. In the center of the cell group, the situation is still good, i.e. the effective current remains within an acceptable range, which in FIG. 2 is represented by the cross-hatched area. On the other hand, problems are detected at the ends of the cells, owing to an effective current that is either too high or too low. From the Figure it can be seen that in the last electrode intervals in the top left-hand corner, and in the first electrode intervals of the bottom right-hand corner, effective current does not flow at all, i.e. the prevailing effective current is 0 A. Now metal is not precipitated on the cathode at all. A deficient layer of precipitated metal on the cathode surface in turn causes problems in the mechanical separation of metal from the permanent cathode. Further, from FIG. 2 it is seen that the effective current in the electrode intervals in the bottom left-hand corner and in the top right-hand corner approaches the top limit 700 A of current distribution. An excessive effective current causes a rapid precipitation of metal on the cathode surface, which can result in short circuits.